Any material, whether natural or synthetic must exhibit satisfactory resistance to degradation under conditions of use, if products made from the materials are to find a lasting market. A lack of satisfactory resistance to degradation usually manifests itself as a partial or total loss of structural integrity, a darkening or discoloration of the product, a loss of flexibility, or a combination of the above phenomena. These phenomena are promoted or catalyzed by air (oxygen), heat and light.
Vinyl halide polymers are particularly susceptible to degradation by heat because of the high temperatures at which these polymers must be processed to make useful articles. Processes such as extruding, calendering and molding are all performed at elevated temperatures. Also there is much exposure to heat in the normal manufacturing processes. After being polymerized, vinyl halide resins are steam stripped of residual monomer and dried, all at elevated temperatures. The steam stripping process normally reaches temperatures well above 100.degree. C.
To protect these vinyl halide resins from heat degradation, it is customary to add materials known as stabilizers. Stabilizers are usually added by the formulaters at the time other ingredients such as processing aids, fillers, colorants and the like are added. This method of adding the stabilizers at the time of compounding is satisfactory, but it leaves the resin unprotected against heat during post polymerization manufacturing operations such as stripping and drying. In some circumstances this delay in adding the stabilizer will result in off-color or dark resin.
Stabilizers are not normally added prior to or during the stripping operation because of a tendency of these materials to contaminate the recovered monomer and to be relatively ineffective when added to a wet resin. Unreacted monomer is captured and recycled to make polymer from the post polymerization operation (blowdown tank, stripper and dryer). If stabilizers are carried over by the recycled monomer, they can adversely effect the later polymerizations.
Many stabilzers have been proposed for vinyl halide polymers and several have enjoyed wide acceptance by the industry. Stabilizers usually fall within one of four classes: (1) inorganic compounds; (2) metallo-organic salts or soaps (sometimes included under the inorganic class); (3) true organometallics containing a carbon-to-metal bond; and (4) pure organic compounds. Many of these stabilizers, although effective, are not approved for applications which come in contact with food and beverages.
It would be desirable to have a vinyl halide resin which is stabilized against heat degradation during post polymerization manufacturing operations and later end uses. It would also be desirable to have a stabilizer which is suitable for use in food grade applications.